In drive mechanisms for linear movement comprising screw and nut devices with a low internal friction, the friction may be so low that the screw and nut system is not self-locking, which is a disadvantage in some respects. The low friction, in e.g. a hoisting device, implies that a load hanging in the device will sink as soon as the external hoisting torque on the screw or nut is disengaged, which for security reasons is unacceptable.
The low internal friction, which can be obtained in e.g. so-called ball screw mechanisms, is desirable for energy conservation reasons, however. When such a device is used, in for example a hoisting device, certain rotation locking means must be used. These means usually comprise a brake provided in connection to the drive mechanism, which is thereby comparatively complicated.
The purpose of the present invention is to provide a simple and efficient locking function in connection to the nut in a drive mechanism according to the introduction, so that a screw and nut device becomes self-locking without having a high internal friction which brakes the forward motion of the device.
In accordance with the present invention, a helical spring is provided in the helical thread around the shaft which is of a cross section to complement the thread surface which has end portions inserted into a space 8. By this arrangement torque transmission takes place by squeezing the end portion of the helical spring against one of the surface portions. A wedging action develops between the spring and the shaft when the spring is squeezed radially to increase the contact pressure and thus the braking action.
The existing parts of a screw and nut drive mechanism are efficiently used in such a device, and the complete arrangement can be compact with but few components.